The present invention generally relates to an ultrasonic step motor, and more particularly, to an ultrasonic motor driven by ultrasonic vibration in the form of an ultrasonic wave.
Conventional types of ultrasonic motors using vibration have been proposed as described in Japanese Patent Application Laid-Open (KOKAI) No. 58-93477, Japanese Patent Application Post-Examination Publication No. 59-37673, and Japanese Patent Application Laid-Open (KOKAI) Nos. 59-122385 and 60-51478. The Japanese Patent Laid-Open No. 58-93477 discloses a method for arbitrarily switching the driving direction of a piezoelectric motor as required by applying a voltage output having a prescribed phased difference to a plurality of vibrators. Japanese Patent Laid Open No. 59-122385 discloses a method for converting strong vibration energy of a supersonic wave into rotational or linear movement. Japanese Patent Laid Open No. 60-51478 discloses a motor which displays accurate time by etching a pattern of sixty divisions on an electronic timepiece made of a piezoelectric motor. The pattern detects the rotating state to control the drive of the motor. All of the conventional ultrasonic motors rotate continuously as long as a driving signal is applied thereto.
The prior art ultrasonic motors have been satisfactory. However, prior art motors utilize a rotor which continuously rotates or rotates only when the driving voltage driving wave shapes are input to the motor. Accordingly, feedback control must be effected in order to control the rotational speed and stop position of the conventional ultrasonic motors. Hence the system of the prior art requires a feedback control circuit resulting in the complication of the entire control system of the ultrasonic motor.
Accordingly, it is desirable to provide an ultrasonic motor which overcomes the shortcomings of the prior art.